The invention relates to a method and apparatus for thermal processing of powder raw materials, more specifically, in the manufacture of cement clinker from cement raw meal, which is preheated in at least one heat exchange line, more specifically a cyclone heat exchange system through which a rotary kiln waste gas flows, calcined in a pre-calcination stage, and fired in the sintering zone of the rotary kiln into cement clinker, which is cooled in a downstream cooler, the rotary kiln waste gas flow supplied with fuel in the pre-calcination stage being used for the pre-calcination of the raw meal, and the waste gas flow of the heat exchange line is optionally fed to waste gas conditioning through a scrubber.
To avoid using an uneconomical long or large-diameter rotary kiln for the manufacture of cement clinker from raw meal and also to minimize the specific energy consumption of the cement clinker manufacturing process, it is known to provide a pre-calcination stage of a rotary kiln. As described, e.g., in EP-B 0 497 937, such a pre-calcination stage is provided between the heat exchange line and the rotary kiln and has at least one additional furnace (in addition to the rotary kiln furnace). Fuel is added in the pre-calcination stage to the rotary kiln waste gas that is contained in the cement raw meal preheated in the heat exchange line, whereby highly calcined cement raw meal is obtained through fuel combustion to be fed to the rotary kiln.
Taking into account emissions of pollutants such as, for example, CO and NOx, it is known to burn fuel in the pre-calcination stage in a quantity below the stoichiometric quantity by providing a CO-containing reducing zone for reducing harmful NOx, which are formed especially in high-temperature fired rotary kilns (thermal NOx). CO that is not consumed in the NOx reduction zone of rotary kiln waste gas ducts and also in the pre-calcination stage, more specifically at the non-burned fuel particles, is after-burned with oxygen of a tertiary air flow supplied from the clinker cooler. This residual burning is facilitated by diverting a flow of a gas-solid suspension in the pre-calcination stage, more specifically, by providing a swirl chamber or mixing chamber in the flow diverting area.
The above-described measures are used to reduce the undesired pollutant emissions that are formed from reaction products and from non-burned fuel components, the harmful emissions occurring directly in the rotary kiln or in the pre-calcination stage that is provided immediately upstream the rotary kiln. Since lower temperatures often cannot be achieved in the further treatment of the waste gas, or they cannot be achieved except at a high cost (e.g., by using activated charcoal filtering), the above-described measures require that the waste gas be treated as close as possible to the point where the pollutants are formed before the waste gas enters the heat exchange line.
However, such treatment in certain cases is not possible, and the pollutants remain in the raw materials. This is the case, e.g., when raw materials that are used for cement clinker manufacture have a high sulfide level or an elevated TOC (total organic carbon) level, e.g., when the raw material contains bituminous shale, and the raw materials are uncontrollably incompletely burned in the upstream cyclone stage, thus resulting in higher emissions of CO, VOC (volatile organic carbon), and S2xe2x88x92 in the waste gas.
To obviate the difficulties encountered in burning such raw materials, according to the invention, it is an object of the invention to provide an economical and efficient method and an apparatus for carrying out same which would allow for using the energy of a raw material having high content of TOC and/or sulfides for cement clinker manufacture without substantial technical and structural problems.
The above object is accomplished according to the invention as far as the method is concerned by directing the entire waste gas flow through an oxidation zone having an excess of oxygen and open flames provided by an afterburner. The features recited in the method regarding the apparatus embodying the principles of the present invention, an oxidation zone with an afterburner is provided in the waste gas duct between an uppermost cyclone stage of the heat exchange line and the scrubber and/or a dust filter. The entire flow of the waste gas that moves through the waste gas duct being directed through open flames of the afterburner.
In a method according to the invention, it is provided that, for complete burnout or oxidation of substances of the waste gas in the heat exchange line, which have high level of CO, S2xe2x88x92, VOC (volatile organic carbon), e.g., hydrocarbons because of respectively high level of TOC (total organic carbon) and/or sulfide in the raw materials, which result in high harmful emissions, the entire waste gas flow is directed through an oxidation zone having an excess of oxygen and open flames produced by an afterburner.
The desired oxidation of harmful materials depends only on the thermal reactions, more specifically, on reactions between the harmful substances and the radicals of the open flames. The waste gas temperature after the afterburning is normally within the range from 450xc2x0 C. to 680xc2x0 C.
To save energy, the afterburner can use conventional fuels such as natural gas or oil. In a preferred embodiment of the invention, it is also possible to use alternative fuels such as waste oil (e.g., PCB-free).
According to the invention, the afterburner can be installed upstream of a scrubber in the path of the waste gas (with the adsorption of S2xe2x88x92, oxidation of VOC and/or CO) and/or downstream of the scrubber for the waste gas that has been pre-cleaned from dust.
For economic burnout with oxidation of the harmful contents of the waste gas, according to another preferred embodiment of the invention, the waste gas is fed into an afterburner duct provided downstream of the afterburner with a residence time there of about 1.5 seconds with swirling, and, to enhance the swirling in the afterburner duct, a swirling chamber is provided, e.g., in the upper turn thereof.
To cool down the waste gas that is heated in the afterburner and to keep constant oxygen excess in the oxidation zone, it is provided according to an embodiment of the invention that fresh air is admitted to an waste gas line immediately downstream of the afterburner. If this cooling is not sufficient, or as an alternative or in addition to this cooling, the temperature of the waste gas that leaves the heat exchange line is lowered before the entry to the afterburning duct, e.g., by providing a cyclone stage as an extension of the heat exchange line. In this case, energy supply from the afterburning can be used in an optimum manner for raw meal drying or for a crusher and drier plant.
Another possibility of cooling the waste gas that has been heated too much in the afterburner resides in matching the scrubber that is installed upstream, more specifically, its cooling capacity, in the event that the oxidation zone incorporates a scrubber.
If the afterburner according to the invention is used in the path of the waste gas flow between the heat exchange line and a dust filter, it is possible to control the temperature of the waste gas that is used for raw meal drying/for the crusher and drier plant in such a manner that this temperature will never exceed the temperature of the heat exchange line and that this temperature will be exactly as per the seasonal requirements for the afterburner. This exact temperature control can be advantageously used to further optimize performance of the crusher and drier plant.
Other advantages, properties, and features of the invention will become apparent from the following description of an illustrated schematic flow diagram of an exemplary embodiment of an apparatus for the manufacture of cement clinker and a schematic representation of an afterburner.